Dwindling oil reserves and soaring oil prices have increased commercial interest in alternative fuels. Coal, oil sands and oil shale, by comparison, are found in higher abundance. These carbonaceous materials can be “liquefied” (ie converted to liquid hydrocarbons), and the produced liquid hydrocarbons can be processed to form many petroleum products such as petrol and diesel, thereby offering an alternative fuel source to traditional oil and oil products.
Coal can be found in a number of different forms determined by its organic maturity, with more mature forms considered to be higher quality or rank. The coal maturation pathway starts with peat, which turns into lignite (or brown coal), which is young, low rank coal. Lignite matures into sub-bituminous coal. Both lignite and sub-bitumous coal are soft, low rank coals characterised by high moisture levels and low carbon content, and accordingly, have low energy values. Higher rank coals such as bituminous coal and anthracite are generally harder and stronger, and have lower moisture content and higher carbon content, and accordingly, have a higher energy value. Graphite is the highest rank coal. Lower ranks of coal are usually found are found closer to the surface, and the rank of coal increases with depth. Industry has primarily been focussed on mining high rank coal that is close to the surface.
Liquid hydrocarbons can also be extracted from oil shale, which is a sedimentary rock that contains significant amounts of kerogen (a solid mixture of organic chemical compounds from which liquid hydrocarbons can be extracted). Liquid hydrocarbons can also be extracted from oil sands (also known as tar sands or bituminous sands), which are a naturally occurring mixture of sand or clay, water and a dense or viscous form of petroleum known as bitumen, which is considered a major source of unconventional oil.
Coal formations are complex and heterogeneous, and mixed ranks of coal, oil shale and/or oil sands are often found in the same coal formation. Coal formations are frequently ingrained with various impurities including mineralisations such as pyrene, pyrite and pyridine. Coal includes “volatile matter” which refers to the components of coal (other than moisture) which are released from coal at high temperature in the absence of air. The volatile matter is usually a mixture of short and long chain hydrocarbons, aromatic hydrocarbons and some sulphur. Chemically, coal has a matrix structure composed mainly of aromatic and hydroaromatic ring compounds containing carbon, hydrogen and oxygen atoms, which form clusters linked by ether or methylene bridges. Conversion of coal to liquid hydrocarbon requires the cleavage of chemical bonds between certain atoms in coal molecules, including the ether or methylene bridges.
Coal can be converted to liquid hydrocarbon by the Fischer-Tropsch Process, or “indirect” processes. In the Fischer-Tropsch process, mined and pulverised coal is first “gasified” (ie converted to a gaseous form) by “pyrolysis” (the term given to decomposition of a substance at very high temperatures) and then liquefied in above-ground purpose built reactors. Mined and pulverised coal is mixed with water to form a coal slurry. The coal slurry is gasified into carbon monoxide and hydrogen gases (a mixture known as synthesis gas or syngas) at high temperature (eg 700-1000° C.) and high pressure, in the presence of a catalyst, and in a carefully controlled oxygen concentration in a gasifier. The “Fischer-Tropsch reaction” then occurs, usually in a reactor, whereby the syngas mixture is reacted in the presence of a catalyst (usually an iron or cobalt catalyst) to produce a liquid hydrocarbon, water and carbon dioxide. The resulting hydrocarbons are then refined to form the desired synthetic fuel.
It is also possible to directly liquefy coal to produce liquid hydrocarbons using the Bergius process. In the Bergius process, mined and pulverised coal is directly liquefied by “hydrogenation”, whereby chemical bonds (eg double bonds between two carbon atoms in a coal molecule) are reduced by a reaction that binds hydrogen atom(s) in above-ground purpose built reactors. Lignite (brown coal) or sub-bituminous coal is finely ground and mixed with heavy oil recycled from the process, in the presence of a catalyst (for example, tungsten, molybdenum sulphides, tin, or nickel oleate catalysts). The mixture is pumped into a reactor, and the hydrogenation reaction occurs at high temperature (eg 400-500° C.) and high pressure (eg 20-70 MPa hydrogen pressure), converting coal to liquid hydrogen in the presence of high pressure gaseous hydrogen.
The above methods of liquefying coal to liquid hydrocarbons do not efficiently utilise coal formations. This is in part because coal formations are complex and the above liquefaction processes tend to utilise high rank coal in coal formations that are relatively easy to access; and further, the coal is mined, pulverised and purified prior to liquefaction, removing many minerals, water, organic compounds and volatiles entrained within the coal formation. Additionally, the costs and input energy required to perform these processes is high relative to the liquid hydrogen product obtained, and further, the environmental footprint of these processes is undesirable.
Recently, there has been interest in the use of supercritical fluids to extract liquid hydrocarbons from coal and other carbonaceous substances such as oil sands and oil shale. A supercritical fluid is a fluid at high temperature and pressure (generally considered to be at or above a “critical temperature” and a “critical pressure”) such that the density of the liquid phase is approximately equal to the density of the gaseous phase resulting in conditions wherein the phase boundary between the liquid and a gaseous phases of the aqueous solution ceases to exist such that there is no (or very little) distinction between the two phases. It has recently been shown that supercritical water can be used to successfully extract liquid hydrocarbons from coal, oil sands and oil shale; however, such research has generally been conducted in above ground reactors that are purpose built to withstand the high pressures, high temperatures and highly solvent properties of a supercritical fluid, and such reactors are necessarily expensive.
The present inventor has realised that due to the complexity of coal formations, it is advantageous to liquefy carbonaceous materials such as coal in situ in a coal formation using a two-step process that initially heats the reaction zone and secondly utilises supercritical, superheated or high-velocity superheated fluids to facilitate liquefaction of a carbonaceous substance by a liquefaction reaction that utilises the natural properties of the coal formation.